The present invention relates to a negative pressure booster to be used as a brake booster, and in particular to a tandem type negative pressure booster, in which two power pistons are arranged in tandem.
In the past, a tandem type brake booster having two power pistons arranged in tandem has been known and disclosed, for example, in the Japanese Provisional Patent Publication No. 55-152656 and the Japanese Provisional Utility Model Publication No. 60-62363, with the purpose of providing higher braking force with lower treading force on brake pedal in a brake booster utilizing negative pressure.
In such a tandem type brake booster, when the brake pedal is pressed for braking, the control valve is switched over. A negative pressure is applied by two power pistons to the two constant pressure chambers and two variable pressure chambers when not in operation. When control valve is switched over, air at atmospheric pressure is introduced into the two variable pressure chambers. By the action of the atmospheric air thus introduced, the two power pistons are operated. By the action of these power pistons, the master cylinder is operated, brake fluid pressure is generated, and braking is performed. In this case, the piston of master cylinder is operated by two power pistons, and maximal braking fluid pressure generated is higher when compared to a the brake booster, which is operated by a single power piston.
Incidentally, electronically controlled components and devices are being increasingly used in recent years for reducing engine room, and there are strong demands for a brake booster, which is shorter in length and compact in design and provides higher output.
In the conventional tandem type brake booster, a valve body or cylinder member mounted on valve body is airtightly and slidably supported on a center plate, which separates the space in the shell into a front chamber and a rear chamber. On the valve body or cylinder member, an inner bead unit having thick front and rear diaphragms is mounted. This leads to a longer axial length of the subassembly where these two inner bead units are mounted on a valve body or cylinder member. Particularly, it is necessary to maintain a predetermined stroke of valve body or cylinder member when inner bead unit is mounted, and it is not possible to shorten its length. As the result, the axial length of brake booster becomes unavoidably long, and its above demand to shorten the overall length cannot be fulfilled.
Also, when inner bead unit is mounted on valve body or cylinder member, a special component and parts such as a fastening member are used. This results in a complicated mounting procedure and its assembly can not be done very efficiently.
Further, in a tandem type negative booster, the two constant pressure chambers permanently communicate with each other through a vacuum passage, and the two variable pressure chambers communicate with each other permanently through an air passage. For example, in the negative booster as disclosed in the above publications, one of these air passage and vacuum passage is furnished on valve body, and the other passage is provided on inner side of the shell.
However, if one of the passages is provided on inner side of the shell, the effective diameter of power piston must be reduced to give space for the passage, and this leads to the reduction of an effective pressure receiving area. The reduction of the effective pressure receiving area increases output loss on the negative pressure booster.